U.S. and Canadian natural gas Class 8 truck retail sales improved in May after getting off to a slow start earlier in the year, according to a recent report from ACT Research.

The “Natural Gas Quarterly” attributes this to a high number of natural gas vehicle repeat sales, as well as purchases by transit bus and refuse truck operators. However, the continuing low cost of diesel is making the return on investment for adopting natural gas less lucrative for fleets not yet invested in NG-fueled vehicles.

“With the fuel price differential continuing to narrow, the ROI to convert from diesel to natural gas is moving in the wrong direction: payback periods remain lengthy,” said Ken Vieth, ACT’s senior partner and general manager. “This doesn’t mean the adoption of NG fuel has stopped or that there are no new developments supporting a future uptick in NG truck orders. Despite a 48% month-over-month uptick in May, year-to-date volumes are 24% below 2015’s level and year-over-year sales are down 21%. NG infrastructure continues to be built, albeit at targeted locations and at a slowing pace. Existing NG equipment users remain committed to its long-term viability and emission benefits.”

Additionally, the report provides examples of how equipment research and development efforts are continuing to advance the market. ACT Research sees only modest, single-digit growth for the adoption of natural gas as a transportation fuel in the U.S. the next few years, barring legislative changes.